In general, thermal insulating materials have been conventionally composed of urethane. However, vacuum thermal insulating materials having a better thermal insulating property than urethane have been recently used with the urethane. Such vacuum thermal insulating materials are used for refrigerators and cold equipments such as heat insulating storage tanks, air conditioners for vehicles, and water heaters.
A vacuum thermal insulating material has a powder, a foam, fibers, or the like serving as a core material contained in a wrapping material composed of aluminum foil and having a gas barrier property (equal to an air shutoff property), whose interior is maintained at a degree of vacuum of several Pa.
Possible causes of degradation of the thermal insulating property of such a vacuum thermal insulating material are air and water entering the vacuum thermal insulating material from outside, gas emitted from the core material, and moisture originally contained in the core material. To absorb these substances, an absorbent is inserted into the wrapping material.
Although as a core material of the vacuum thermal insulating material, powder such as silica, foam such as urethane, and fiber body such as glasses are available, the fiber body having the most superior thermal insulating performance becomes a mainstream nowadays.
Fibers are broadly divided into two types: inorganic fibers and organic fibers. Examples of inorganic fibers include glass fibers and carbon fibers (for example, see Patent Documents 1 and 8). Examples of organic fibers include polystyrene fibers, polypropylene fibers, polylactide fibers, aramid fibers, LCP (liquid crystal polymer) fibers, polyethylene terephthalate fibers, polyester fibers, polyethylene fibers, and cellulose fibers (for example, see Patent Documents 2, 7, and 9).
Such fibers are arranged in a shape (form) of, for example, a flocculent form, a stack of sheets (for example, see Patent Documents 3 and 4), or a stack of sheets in which the sheets are stacked such that the fibers are alternately oriented in different directions (for example, see Patent Documents 5 and 6).